Hi, I Recently Have Been Bruising In Several Places On

Hi, I recently have been bruising in several places on my arms and legs. Now, I am an active 17 year old teenage girl and I can see where I could just be bumping into objects and bruising without realizing it, however I have counted 5 large bruises on my legs and am beginning to get very concerned. I am also tired and sore everywhere around my body and when I'm sitting in a chair I feel short of breath. Not gaining or losing weight, not sexually active, and have had extremely heavy periods to the point where I can't go to school sometimes. Periods last 5 days (always 5) but are extremely heavy and the cramps are so bad that no medication can help. This has been going on since around August. Please help.